Onion (Allium cepa L.) is a biennial crop that in temperate regions is planted in the spring and, after a juvenile stage, forms a bulb in response to the lengthening photoperiod of late spring/ summer. The bulb then overwinters and in the next season it flowers and sets seed. FLOWERING LOCUS T (FT) encodes a mobile signaling protein involved in regulating flowering, as well as other aspects of plant development. Here we show that in onions, different FT genes regulate flowering and bulb formation. Flowering is promoted by vernalization and correlates with the upregulation of AcFT2, whereas bulb formation is regulated by two antagonistic FT-like genes. AcFT1 promotes bulb formation, while AcFT4 prevents AcFT1 upregulation and inhibits bulbing in transgenic onions. Long-day photoperiods lead to the downregulation of AcFT4 and the upregulation of AcFT1, and this promotes bulbing. The observation that FT proteins can repress and promote different developmental transitions highlights the evolutionary versatility of FT.

No abstract

In climates that experience short growing seasons due to drought, heat, or end-of-season frost, early flowering is a highly desirable trait for chickpea (Cicer arietinum). In this study, we mapped, sequenced, and characterized Early flowering1 (Efl1), an ortholog of Arabidopsis (Arabidopsis thaliana) EARLY FLOWERING3 (ELF3) that confers early flowering in chickpea. In a recombinant inbred line population derived from a cross between CDC Frontier and ICCV 96029, this gene was mapped to the site of a quantitative trait locus on Ca5 that explained 59% of flowering time variation under short days. Sequencing of ELF3 in ICCV 96029 revealed an 11-bp deletion in the first exon that was predicted to result in a premature stop codon. The effect of this mutation was tested by transgenic complementation in the Arabidopsis elf3-1 mutant, with the CDC Frontier form of CaELF3a partially complementing elf3-1 while the ICCV 96029 form had no effect on flowering time. While induction of FLOWERING LOCUS T homologs was very early in ICCV 96029, an analysis of circadian clock function failed to show any clear loss of rhythm in the expression of clock genes in ICCV 96029 grown under continuous light, suggesting redundancy with other ELF3 homologs or possibly an alternative mode of action for this gene in chickpea. The 11-bp deletion was associated with early flowering in global chickpea germplasm but was not widely distributed, indicating that this mutation arose relatively recently.

The molecular pathways responsible for the flowering response to photoperiod have been extensively studied in Arabidopsis thaliana and cereals but remain poorly understood in other major plant groups. Here, we describe a dominant mutant at the LATE BLOOMER2 (LATE2) locus in pea (Pisum sativum) that is late-flowering with a reduced response to photoperiod. LATE2 acts downstream of light signaling and the circadian clock to control expression of the main photoperiod-regulated FT gene, FTb2, implying that it plays a primary role in photoperiod measurement. Mapping identified the CYCLING DOF FACTOR gene CDFc1 as a strong candidate for LATE2, and the late2-1D mutant was found to carry a missense mutation in CDFc1 that impairs its capacity to bind to the blue-light photoreceptor FKF1 in yeast two-hybrid assays and delays flowering in Arabidopsis when overexpressed. Arabidopsis CDF genes are important negative regulators of CONSTANS (CO) transcription, but we found no effect of LATE2 on the transcription of pea CO-LIKE genes, nor on genes in any other families previously implicated in the activation of FT in Arabidopsis. Our results reveal an important component of the pea photoperiod response pathway and support the view that regulation of FTb2 expression by photoperiod occurs via a CO-independent mechanism.

Transient expression is a powerful method for the functional characterization of genes. In this chapter, we outline a protocol for the transient expression of constructs in Medicago truncatula leaves using Agrobacterium tumefaciens infiltration. Using quantitative real-time PCR we demonstrate that the infiltration of a construct containing the LEGUME ANTHOCYANIN PRODUCTION 1 (LAP1) transcription factor results in the strong upregulation of key biosynthetic genes and the accumulation of anthocyanin pigment in the leaves after just 3 days. Thus, this method provides a rapid and powerful way to the discovery of downstream targets of M. truncatula transcription factors.